refactor: self-referential type annotations for self-referential ContinuedFraction methods

[sr-murthy]

ATM the type annotations for ContinuedFraction class or instance methods that actually return ContinuedFraction objects are incorrect - the current return type for these is fractions.Fraction, whereas in fact they should be ContinuedFraction. To enable this a future statement is required for type annotations:

from __future__ import annotations

This is also a problem for the ContinuedFraction.validate class method and the ContinuedFraction.__init__ method, which need to be amended to accept valid input combinations involving ContinuedFraction objects.

The methods that need to be fixed are:

• ContinuedFraction.validate
• ContinuedFraction.__new__
• ContinuedFraction.from_elements
• ContinuedFraction.__init__
• All the rational operational methods, e.g. __add__, __radd__, __sub__, __rsub__, etc.

Examples in docstrings and the documentation should be updated.